Objective: This in vitro study determined the combination effect of nitrate and lactate on nitrate reduction, rumen fermentation and methane production. We hypothesized that the combination decreases methane production and enhances nitrate reduction.Methods: An in vitro batch culture was conducted to determine the 2 main factors of nitrate and lactate in 2×2 factorial arrangement: a basal diet (CON), the basal diet supplemented with nitrate (2.2% in dietary dry matter [DM]; Nit), lactate (2.2%; Lact), or the combination of nitrate and lactate (Nit-Lact). Rumen fluids from lactating Holstein cows were obtained and mixed with McDougall’s buffer for the in vitro batch culture. Total gas production, methane production, pH, volatile fatty acid (VFA), NH3-N, NO3-N and NO2-N concentration, and DM digestibility were determined.Results: A trend of interaction between nitrate and lactate was observed for DM digestibility (p = 0.06), which occurred because DM digestibility tended to decrease for Nit compared with CON (42.2 vs. 45.3%) but did not for Nit-Lact. Nitrate decreased (p<0.05) methane (25.6 vs. 31.0 mL at 24 h) and total gas production (296 vs. 313 mL at 24 h). Total VFA production was not affected by treatments. However, nitrate increased the proportion of acetate (p<0.01) and decreased (p<0.01) proportions of propionate, butyrate and valerate at 24 h of incubation. However, lactate increased valerate at 24 h of incubation (p<0.01). The concentration of NO3-N for Nit-Lact was lower than that of Nit (41.2 vs. 51.0 mg/L; p<0.01) at 3 h of incubation.Conclusion: Nitrate decreased methane production. The combination of nitrate and lactate had beneficial effects on DM digestibility and nitrate reduction, compared with nitrate alone although no interaction between nitrate and lactate was shown for VFA production and methane production. Changes in the major VFA by nitrate indicated that nitrate acted as an alternative electron acceptor to decrease methane production.

The objective of this study was to develop a facemask system and evaluate its potential for measuring enteric CH₄ and CO₂ production. Recovery tests were conducted and resulted in 99.0% (SD, 3.2) and 102.3% (SD, 2.6), respectively. An experiment was conducted and the facemask system was compared with the GF system. Five mid-lactation cows were fed a lactation diet ad libitum (Period 1) for 5 days and then fed the same diet at 90% of the ad libitum intake for 5 days (Period 2) where CH₄ and CO₂ productions were measured at the end of each period. While daily CH₄ and CO₂ productions were different (p ≤ 0.04) between Period 1 and 2, those did not differ between the methods, and no interaction between period and method was observed. A linear regression analysis indicated a good correlation for CH₄ (CCC = 0.72; RMSE = 22% of the mean) and CO₂ (CCC = 0.79; RMSE = 10% of the mean) between the facemask and the GF system, while systemic bias was observed for CO₂. In conclusion, the facemask system developed has the potential to measure CH₄ and CO₂ production from lactating cows and the measurement was similar with that from the GF system.